1. Field of the Invention
The present invention relates to disk drives for computer systems. More particularly, the present invention relates to a disk drive performing multi-level prioritization of entries in a suspect sector list for identifying and relocating defective data sectors.
2. Description of the Prior Art
When manufacturing a disk drive, servo sectors 20-2N are written to a disk 4 which define a plurality of radially-spaced, concentric data tracks 6 as shown in the prior art disk format of FIG. 1. Each data track 6 is partitioned into a plurality of data sectors wherein the servo sectors 20-2N are considered “embedded” in the data sectors. Each servo sector (e.g., servo sector 24) comprises a preamble 8 for synchronizing gain control and timing recovery, a sync mark 10 for synchronizing to a servo data field 12 comprising coarse head positioning information such as a track number for use during seeking, and servo bursts 14 which provide fine head positioning information for use during tracking. In a “headerless” format, the servo data field 12 also includes information identifying the data sectors between the servo sectors. Each data sector similarly comprises a preamble and a sync mark for synchronizing to a user data field.
A media defect may render one or more data sectors unusable since data written over a media defect may be unrecoverable. In addition, defects in the servo sectors may corrupt the sector identification information or interfere with the tracking operation thereby rendering all or part of a data track unusable. During an Intelligent Burn-In (IBI) phase of the manufacturing process the disk drive is tested rigorously in an attempt to identify all of the unusable (defective) data sectors so that they are not used during normal operation. Invariably a number of suspect data sectors will pass the manufacturing IBI but then identified as too unreliable during normal operation and therefore relocated to spare data sectors. Media defects may also manifest during the life of the disk drive due, for example, to particles contaminating the surface of the disk, the head striking the surface of the disk, or other degradation.
Prior art disk drives execute an off-line scan to scan all of the usable data sectors in a background operation in an attempt to identify and relocate defective data sectors. The off-line scan typically involves reading data from each data sector, the data having been written during IBI or while in-the-field during normal operation. If heroic recovery techniques (e.g., retries, firmware error recovery, etc.) are needed to recover a data sector, the data sector is rewritten and read again. If after a number of write/read operations the data sector cannot be recovered reliably, the data sector is deemed defective and relocated to a spare sector. The data sector is written/read a number of times to ensure that an inability to recover the data sector is due to a media defect rather than a transient write problem. This helps ensure only truly defective data sectors are relocated since performance degrades as the number of relocated data sectors increases, however, it also increases the time required to verify a data sector. Consequently scanning the entire disk surface can require a significant amount of time, particularly as the number of tracks per inch (TPI) increases with higher capacity disk drives as well as for applications where the disk drive is operating frequently with only minimal idle time available for the off-line scan.
There is, therefore, a need for a more intelligent off-line scan capable of identifying and relocating defective data sectors in a disk drive to minimize catastrophic data loss.